1. Field of the Invention
The present invention relates to a suspension coil spring which is employed for a suspension device for a vehicle such as an automobile, and more particularly, to a suspension coil spring which is suitably applicable to a strut-type suspension device.
2. Background Art
In recent years, a strut-type suspension device which is widely used as a suspension device for a vehicle is of a type in which a shock absorber is used as a positioning support (i.e., strut) with respect to a wheel. The shock absorber as a strut comprises a cylinder, a rod which is slidably supported by the cylinder, and a compressive coil spring which is disposed at an outer circumferential side of the rod. In this strut-type suspension device, an upper end portion of the rod is connected to a vehicle body via a strut mount or the like, and a lower end portion of the cylinder is rigidly connected to a knuckle which supports a wheel rotatably. Here, the knuckle is pivotally connected to the vehicle body via a lower arm. Further, the compressive coil spring (simply referred to as a “coil spring”, hereinafter) is placed between an upper spring seat, which is fixed at a vehicle body side, and a lower spring seat, which is fixed at an outer circumferential surface of the cylinder, so as to be in a compressed state, and then supported at an outer circumferential side of the rod.
As compared to other separate-type suspension devices, the strut-type suspension device as described above has merits that the required number of parts is reduced, the required structure is less complicated, and the required space for installation is small. However, since a strut axis and a load-input axis (axis that connects a road holding point of a tire and an upper mount point of a strut) are displaced from each other, a bending moment occurs at the strut. The bending moment causes a force forcibly acting against another force to a piston and a bushing which form a sliding section of the shock absorber. Accordingly, a frictional amount of the rod increases thereby causing smooth operation of the shock absorber to be hindered, whereby a vehicle riding quality is deteriorated. At present, in order to lessen the occurrence of such a bending moment as described above, there have been proposed a method in which the coil spring is mounted to the suspension device so as to be offset from the strut axis to thereby cancel the bending moment and a method in which a sliding surface of a bearing portion or a piston portion in the strut is made from a low frictional material.
However, in the strut-type suspension device, sine the larger the tire widths of a vehicle, the more outwardly the road holding point of the tire moves, it actually becomes impossible to offset the coil spring so as to position an operating line of a spring reaction force (spring reaction axis) coincident with or sufficiently close to the load input axis. Meanwhile, even when the tire width of the vehicle is not so large, in order to prevent the coil spring from interacting with the vehicle body, it is preferable that an offset amount of the coil spring is small.
From the aforementioned viewpoints, for example, Japanese Patent Application (JP-A) Laid-Open No. 2000-104772 discloses a suspension coil spring in which a compressive coil spring whose coil axis in a free state is substantially bent at a predetermined curvature, and a pitch of each of a lower end turn portion and an upper end turn of the spring coil spring is set such that at least one of a lower seating surface and an upper seating surface which are respectively seated on an upper seat and a lower seat of a suspension device, is inclined in a predetermined direction and at a predetermined angle with respect to the lower seat and the upper seat. In accordance with this suspension coil spring, an inclination and a vehicle width directional position of a spring reaction axis can be controlled by controlling the inclination of the lower seating surface or the upper seating surface of the suspension coil spring with respect to the lower seat or the upper seat or a bending amount (shell-bending amount) of the coil axis. Consequently, without requiring an increase of an offset of the coil spring with respect to a strut axis, the spring reaction axis of the suspension coil spring can be positioned coincident with or sufficiently close to the load input axis, thus making it possible to decrease the frictional amount of a shock absorber, and facilitate the operation of the shock absorber. In other words, by controlling the inclination and the shell-bending amount of the lower seating surface or the upper seating surface, respectively, a transverse reaction force is caused by the coil spring. A moment due to the transverse reaction force is made to resist a bending moment of a strut. Accordingly, the transverse reaction force that acts on a sliding section, which comprises a piston, a bushing or the like in the shock absorber, can be mitigated. Further, by controlling a position of the spring reaction axis of the coil spring so as to pass through the center of an upper mount (strut mount), occurrence of friction due to a force forcibly acting against another force at the bearing portion can effectively be prevented. Accordingly, steering performance can be improved.
However, in designing the suspension coil spring disclosed in JP-A No. 2000-104772, since extra shell bending amount and extra end turn pitch which may affect spring characteristics are added as design parameters, a problem is caused in that the design of the coil spring becomes complicated. Further, since an end turn pitch i.e., a degree and a direction of the inclination of each seating surface of the coil spring affects a magnitude and a direction of the transverse reaction force quite sensitively, the coil spring must be manufactured with a considerably high dimensional accuracy. For example, even when a degree or direction of the inclination of the seating surface is slightly different from a design target value of the coil spring, a possibility may occur that required spring characteristics cannot be obtained. For this reason, extra equipment is also required for equipment for manufacturing the coil spring in order to provide the coil spring with a high dimensional accuracy, or extra production management during a manufacturing process of the coil spring. Consequently, a problem is caused in that a manufacturing cost of the coil spring becomes extremely high.
Characteristics of the suspension coil spring as described above is ordinarily analyzed by a non-linear analysis using a finite element method, and on the basis of the results of the analysis, the suspension coil spring is designed. In other words, as long as modeling is performed, characteristics that are newly required, of the suspension coil spring can be analyzed by using finite element codes in general use. However, in the analysis by the finite element method (FEM analysis), spring characteristics can be obtained by inputting spring data and boundary conditions; however, inversely, spring data for satisfying the characteristics that is required by a designer of the suspension coil spring cannot be determined. Accordingly, a repetitive calculation must be done while the spring data is being varied until the results of the analysis which suffices target characteristics can be obtained.
On the basis of a flow chart which is shown in FIG. 13, a specific description will be made of a method of designing the suspension coil spring using the finite element method. First, in step 300, a size and a shape of the suspension coil spring are respectively presumed. Thereafter, in steps 302 to 306, the FEM analysis is performed by combining the presumed size and shape, and boundary conditions. Then, it is determined whether or not results of the analysis satisfy the desired characteristics. At this time, if the results of the analysis satisfy the required characteristics, the size and shape of the suspension coil spring will be determined on the basis of the analysis results. On the other hand, if the results of the analysis do not satisfy the required characteristics, operation returns to step 300, where a repetitive calculation is carried out, during which presumable size and/or shape of the suspension coil spring are varied until the results that satisfy the required characteristics are obtained. However, when the suspension coil spring is designed by using the above-described designing method, there is no way but relying upon the designer's perception or experiences in presuming the size and shape of the suspension coil spring. Accordingly, a problem is caused in that it depends on the degree of skill of the designer of the suspension coil spring whether or not an optimal solution can be figured out.
In view of the aforementioned facts, an object of the invention is to provide a suspension coil spring in which, with the suspension coil spring assembled to a suspension device, a spring reaction axis can be positioned coincident with or sufficiently close to a load input axis, and the design and manufacture of the coil spring is facilitated.